The present invention relates in general to field-effect transistors ("FETs") and in particular to an FET having a drift region adjacent to a channel formed between a gate electrode and a double-diffused back gate.
A junction-field-effect transistor ("JFET") or a metal-semiconductor field-effect transistor (MESFET) comprises a semiconductor channel separating ohmic source and drain regions such that a positive potential between drain and source causes electrons to flow from source to drain. A gate electrode forms a rectifying junction with the channel causing an insulating depletion region to extend from the junction into the channel. In a JFET the gate electrode is semiconductor material and forms a pn junction with the channel, while in a MESFET the gate electrode is metallic and forms a metal-semiconductor junction with the channel. In either case, as the rectifying junction is increasingly reverse biased, or as the drain-to-source voltage increases, the depletion region extends farther into the channel, narrowing the portion of the channel that can support drainsource current and increasing its resistance. Thus the resistance of the channel is a function of both the gate and drain potentials. Channel resistance is also affected by the resistivity of the semiconductor material forming the channel and by the dimensions of the channel, including its "length" in the direction of current flow between the drain and source, its "depth" perpendicular to the direction of current flow and perpendicular to plane of the rectifying junction, and its "width" perpendicular to the direction of current flow and parallel to the plane of the rectifying junction.
The frequency at which a field-effect transistor can operate depends primarily on the mobility of the electrons in the channel material and on the length of the channel. Thus to maximize operating frequency, the channel length should be as short as possible. However, channel lengths in field-effect transistors of the prior art are determined by the dimensions of masks utilized in their fabrication, and dimensional tolerance with which these masks can be fabricated makes it difficult to produce field-effect transistors with very short channel lengths.
Even when field-effect transistors with short channel lengths can be fabricated, the channel length must be larger than the channel depth in order to provide for adequate gate control over current flow. Consequently, as the gate channel length is decreased to permit higher frequency operation, the channel depth must also be decreased. But the reduction in channel depth increases channel resistance at all levels of drain potential and reduces the drain current swing in response to gate potential swing. Since the "power" of a transistor is proportional to the product of the maximum drain voltage swing and maximum drain current swing that it can handle, an increase in channel resistance reduces transistor power.
Channel resistance can be decreased by increasing the doping level within the channel region, but as the doping level increases, the breakdown voltage of the field-effect transistor decreases. The breakdown voltage is the maximum drain voltage that can be tolerated without breakdown of the depletion region, and the breakdown voltage places an upper limit on the drain voltage swing. While increasing doping of the channel increases the drain current swing, thereby tending to increase transistor power, at some point the decrease in breakdown voltage more than offsets the increase in drain current. Thus the power handling capability of a short channel, high frequency field-effect transistor is limited by its low breakdown voltage.
Power handling capability of a short channel field-effect transistor is further limited when high gate input impedance is to be maintained. Field-effect transistors are normally operated in saturation where electrons pass at high velocity through a portion of the channel in which the electric field is high, and some of the high velocity electrons collide with and ionize atoms of the semiconductor material to produce electron-hole pairs. The electrons migrate to the drain, but the holes migrate to the gate, thereby increasing gate "leakage current" and decreasing gate input impedance. The electric field developed in the channel, and therefore the ionization rate, increase with channel doping and drain voltage. While heavy impact ionization produces avalanche breakdown at the breakdown voltage, at lower drain voltages impact ionization can result in an intolerable gate leakage current. Thus, in a high frequency field-effect transistor, the high doping of the channel limits the drain voltage that the field-effect transistor can handle without drawing a large gate current, and therefore places another restriction on the power that the transistor can handle.
The power of a short channel field-effect transistor may also be limited by the "punch-through" phenomena. As the drain voltage increases above pinchoff, the depletion region grows toward the source. When the depletion is near the source, majority carriers in the source region can be injected by thermal activity into the depletion region and then swept by the field therein to the drain. Thus, when the drain voltage is above a "punch-through" voltage, drain current can increase rapidly. For a short channel field-effect transistor, the punch-through voltage is typically below the breakdown voltage and therefore in a short channel field-effect transistor, the punch-through voltage has a more limiting effect on the field-effect transistor's power handling capability than the breakdown voltage.